Japanese Patent Application Laid Open No. 53-83938 describes a plating layer of metal comprising a nickel-phosphor matrix and lubricative fine particles dispersed in the matrix. It also describes sericite and graphite as examples of lubricative fine particles. However, it does not describe any process for producing a plating solution which contains dispersed fine graphite particles.
Japanese Patent Application Laid Open No. 7-102383 describes a nonelectrolytic plating layer of Ni--P metal containing dispersed fine particles. It also describes polytetrafluoroethylene, graphite and molybdenum disulphide as examples of dispersed fine particles. However, it does not describe any practical process for producing the plating solution which contains dispersed fine graphite particles.
Generally, fine particles of graphite are in a form of soot, that simply floats on the surface of a solution and would not be dispersed in the plating solution. While not described in Japanese Patent Applications as explained above, fine particles of graphite can be dispersed into plating solution by using a surface active agent. However, according to the knowledge of the present inventors, plating solution containing usual surface active agent generates a big internal stress in the plating layer, and arises the resultant problem of interlayer exfoliation of the plating layer.
The inventors of the present invention had filed a patent application in Japan on a novel process of forming a Ni plating layer on Sep. 3,1997. According to this prior invention of the present inventors, special composite paticles in which Ni particles are placed on a surface of the graphite particle are previously produced. The special composite particle are added to the plating solution containing Ni, instead of adding usual graphite particles.
With this process, a plating solution containing dispersed fine particles of graphite can be obtained without using a surface active agent. These special composite particles can easily become suspended and dispersed in the plating solution containing Ni, since Ni particles are placed on the surface of the graphite particle. Thus the central particles of graphite are dispersed well into the plating solution by way of the surface particles of Ni without using any surface active agent. The plating layer is free from the problem of interlayer exfoliation.
The plating layer formed by using the solution containing special composite particles is highly lubricative due to the dispersed fine graphite particles. However, it is insufficient in its abrasion-resistant property.
FIG. 1 is a graph made by the present inventors obtained by experiments in which each solution has an identical Ni--P content. According to the findings of the present inventors, the Ni--P plating layer formed by using the solution containing special composite particles softens the matrix and increases the abrasion of the plating layer compared with a Ni--P plating layer formed by using a solution containing dispersed fine particle of BN. In other words, when special composite particles are used, the matrix of the plating layer apt to contain less P so that its final P content remains low to make it difficult to produce a sufficiently hard matrix.
As described above, without special means, fine particles of graphite can hardly be dispersed into a plating solution. They may be dispersed well into a plating solution by using an ordinary surface active agent,then there arises a problem of interlayer exfoliation. On the other hand, they may be dispersed well into a plating solution without using a surface active agent when a plating solution contains special composite particles, then there arises a problem of poorly abrasion-resistant matrix.
It is, therefore, an object of the present invention to provide a sliding member having a plating layer that contains dispersed fine particles of graphite but free from the problem of interlayer separation and the plating layer is sufficiently abrasion-resistant.